1. Field of the Invention
This invention relates to the field of urea-sulfuric acid reaction products, and particularly to methods of converting concentrated urea-sulfuric acid reaction products of one composition to products of differing urea/sulfuric acid weight ratio. These methods permit the conversion of urea/sulfuric acid reaction products of essentially any composition to products of essentially any other composition with sufficient control of reaction parameters to consistently maintain a predetermined product composition and crystallization temperature while avoiding incipient product and/or reactant decomposition and potentially explosive autocatalytic decomposition associated with the more highly exothermic conversions. The invention also relates to the direct production of the desired product from urea, sulfuric acid, and optionally water, simultaneously with the inverconversion of the feed solution, and to the use of a unique direct air heat exchange process for cooling the reacting liquid phase without significant emissions to the atmosphere.
2. Description of the Prior Art
Urea is widely used as a topical, sub-surface and foliar fertilizer. Sulfuric acid has also been widely used in the agricultural industry and in other industries for numerous purposes. It is known to be highly corrosive both to metals and animal tissue, including human skin. In the agricultural industry, sulfuric acid has been used as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of undesired vegetation, and as a selective herbicide on resistant crops such as onions and garlic.
Previous investigators have observed that urea and sulfuric acid can be reacted to form products containing mono- and/or diurea sulfates. This reaction is highly exothermic, which makes it difficult to control reaction temperature in large volume production plants with available methods. The exotherm makes it essentially impossible to control reaction temperature in the formulation of the higher acid content compositions, e.g., where the ratio of sulfuric acid to urea is about 1 or greater. Furthermore, previous investigators did not recognize either the magnitude or importance of incipient product and/or reactant decomposition, or the temperatures at which such decomposition occurs for products having different urea/sulfuric acid ratios. Their methods were not adequate to avoid incipient decomposition, particularly in the higher acid compositions, and they did not recognize the effect of such decomposition on process control or product quality.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. and 25.degree. C. but, as in the case of du Toit, did not develop or disclose methods capable of handling the high exothermic heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acid. I. Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS, 56, 549-53 (1934).
In the article "Adding Plant Nutrient Sulfur to Fertilizer", Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form complexes which are useful in fertilizers.
Jones, in U.S. Pat. No. 4,116,664, disclosed what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 225.degree. F., and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast, the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred.
Other writers have discussed methods for controlling the heat of reaction in highly exothermic systems such as the urea-sulfuric acid reaction described in du Toit, Dalman, and Jones. For instance, William Lohry, "Techniques of Manufacturing Hot Mix Suspensions", National Fertilizers Solutions Association "Round-Up Papers", pages 34-38 (1968), discloses that the exothermic heat of reaction of ammonia with concentrated phosphoric acid can be controlled by either internal or external cooling of the reactants in the reaction vessel, and that it is usually desirable to provide a heel of reaction product in the vessel before adding reactants to prevent drastic variations in product pH.
In U.S. Pat. No. 1,884,105, H. C. Moore discloses a method for producing salts of sulfuric acid, e.g., by reacting concentrated sulfuric acid with liquid anhydrous ammonia, in which control of the highly exothermic reaction is assisted by adding an initial inventory of product to the reaction zone before reactant addition. In Moore's process the total amount of sulfuric acid to be reacted is mixed with a quantity of ammonium sulfate previously produced and that mixture is then reacted with liquid anhydrous ammonia.
Similarly, in U.S. Pat. No. 3,459,499, G. C. Mullen, Jr., discloses a process for ammoniating superphosphoric acid in which, according to Mullen, temperatures are effectively controlled, and diminished product quality associated with excessive reaction temperatures is avoided, in part, by providing, in the reaction zone, a large body of ammonium phosphate product solution followed sequentially by the introduction of the relatively small quantities of ammoniating fluid and phosphoric acid.
Although these investigators disclosed several characteristics of urea-sulfate combinations, methods of making those combinations and, in general, methods of controlling exothermic reactions, they did not recognize either the magnitude or significance of the incipient decomposition temperature in large volumes of reacting urea and sulfuric acid. Nor did they appreciate that incipient decomposition temperature varies with composition, i.e., with the ratio of urea, sulfuric acid and water in the reaction phase, or the effect that exceeding the incipient decomposition temperature has on product composition. They also did not devise or appreciate the need for process conditions required to achieve acceptable reaction rates in large volumes of reacting urea and sulfuric acid while preventing either gross or localized overheating to temperatures in excess of the incipient decomposition temperatures.
The prior art also did not disclose that concentrated urea-sulfuric acid reaction products can be interconverted to products having different concentrations of urea, sulfuric acid or water. Nor did it teach procedures by which such interconversions could be achieved efficiently or the advantages that product interconversion affords in the manufacture of these compositions.
Methods of producing concentrated urea-sulfuric acid reaction products directly from urea, sulfuric acid and, optionally, water are disclosed in my copending application, Ser. No. 318,629, filed Nov. 5, 1981, the disclosure of which is incorporated herein by reference.
It has now been discovered that concentrated urea-sulfuric acid reaction products, such as those disclosed in said copending application, can be interconverted to products of different composition by the methods disclosed herein.
It is therefore one object of this invention to provide an improved method for the production of urea-sulfuric acid reaction products.
Another object of this invention is to provide a method for converting urea-sulfuric acid reaction products to products of different composition.
Yet another object is the provision of a method for interconverting urea-sulfuric acid reaction products to products of different composition in relatively large volumes while maintaining reaction temperature below the product and reactant incipient decomposition temperature, and maintaining the reactant composition and product crystallization temperature at predetermined values.
Yet another object of this invention is the provision of a method for autogenously maintaining a desired reaction temperature during the interconversion of urea-sulfuric acid reaction products to products of different composition in which the interconversion per se is not sufficiently exothermic to maintain reaction temperature.
Another object of this invention is the provision of a continuous method for interconverting urea-sulfuric acid reaction products to products of different composition.
Yet another object is the provision of an improved method for removing excess heat from, and thus maintaining the desired reaction temperature of the reaction phase in the conversion of urea-sulfuric acid reaction products to products of different composition.
Another object of this invention is the provision of a method for the interconversion of urea-sulfuric acid reaction products to products of different composition free of reaction by-products.
Yet another object of this invention is the provision of a method for the interconversion of urea-sulfuric acid reaction products to products of different urea/sulfuric acid ratio that increases plant production capacity.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings and the appended claims.